This invention relates to multimode modems.
It is desirable for a modem to be able to interwork with V.32 and with V.22bis and V.22 modems on the public switched telephone network. It is also desirable that the modem automatically select the data rate and mode in which to operate. Further, any interworking procedure of the multimode modem should be compatible with various implementations of both standards.
An interworking procedure has been described which, for the multimode answer modem, involved sending the initial tone for the V.32 procedure (AC) and the initial tone for the V.22bis procedure (UNSCR1 to 1200 bps) simultaneously. See, "A Proposed Appendix to Recommendation V.32: Interworking Procedure With V.22bis and V.32 Modems", delayed contribution to D-30 to CCITT Study Group XVII, April 1986.
As described in this D-30 document, handshaking tones for both V.32 and V.22bis modulation modes are transmitted; because the total power is restricted, the tones for each modulation method are less powerful than the normal single-mode interworking. This power restriction poses a potential problem after a multimode modem has detected the modulation method of the other modem and needs to remove the other tones and increase the power of the correct signalling tones. A modem designed to V.22bis or V.32 and not to operation with a multimode procedure as described in the D-30 document might not operate properly through this transition. Further, a modem constructed to conform only to V.32, might not reject the additional tones used by a multimode modem to sense for a V.22bis modulation mode. As such, there is a possibility that a multimode modem built upon the algorithm described in the document D-30 might not operate with some implementation of V.32.